An integral part of any paging system is the paging transmitter. The paging transmitters of a paging system are located at geographically spaced apart locations to supply broadcast coverage to a wide geographic area. The details of a paging system can be found in U.S. Pat. No. 5,481,258 to Fawcett et al., U.S. Pat. No. 5,365,569 to Witsaman et al. and U.S. Pat. No. 5,416,808 to Witsaman et al., commonly assigned to the assignee of the present invention and incorporated herein by reference.
A paging transmitter includes certain important primary components, such as the power amplifier, the exciter, and the transmitter controller. The present invention relates to the exciter of a paging transmitter. The exciter is the portion of the paging transmitter that receives data to be encoded (or more accurately termed modulated) onto a carrier frequency suitable for radio-frequency (RF) transmission.
For simple types of modulation, such as amplitude modulation (AM) or frequency modulation (FM), the modulation is often applied directly to the final carrier frequency, without any intervening steps. For more complex types of modulation, however, it is often more convenient for various reasons to apply the modulation to a lower, intermediate frequency (IF). The IF is then upconverted, or shifted up to the desired transmission frequency, using a frequency mixer and a local oscillator (LO).
In the process of upconversion, several undesired frequency byproducts are created along with the desired transmission frequency of the carrier. Among these are the LO frequency, the image of the desired transmission frequency reflected about the LO frequency, and perhaps intermodulation products of these other byproducts against one another. These byproducts are generally eliminated by passing the modulated carrier transmission frequency through a frequency selective filter.
Especially in digital transmitter systems, due to limitations on how quickly the data can be processed, the first IF may be quite low compared to the desired transmit frequency. This leads to impractical filtering requirements, as the desired transmission frequency, the LO frequency, and reflected image are too close in frequency to one another. The filter requirement can be made practicable by employing an additional IF and upconversion process, using a frequency between the first IF and the desired transmission frequency. This type of system is suitable for a paging transmitter that is adapted to broadcast at transmit frequencies spanning less than an octave.
However, it is desirable that a paging transmitter have the flexibility to broadcast at carrier frequencies that span up to two octaves, for example from 130-512 MHz. A paging transmitter that is frequency agile over a wide range is important for uniformity of manufacturing purposes and for providing a paging system capable of broadcast over several bands.
The primary difficulties in designing a paging transmitter with such a wide broadcast range are twofold. First, there may be significant difficulty in filtering away the upconversion byproducts. Second, the LO must have a wide tuning range of frequencies. To illustrate, to have an exciter span the 130-512 MHz range, the LO would have to be tunable from 40-422 MHz or from 220-602 MHz (assuming that a first IF of 90 MHz is used). In the first range, the LO would have to be tunable over a decade and in the second range, the LO would have to be tunable over nearly two octaves. LO's with this frequency agility, in this frequency range, are impractical for paging transmitter applications.
One prior art attempt to solve this problem is to use multiple LO's for each of the possible broadcast carrier frequencies. However, this would typically include higher costs for the multiple LO's. Moreover, because the LO and image frequencies would sometimes fall within the desired transmit band, an output bandpass filter that is retunable on-the-fly, or a switchable bank of such filters, would also be needed if fast channel-change times are required. This too is impractical.
The present invention solves these and other problems of the present invention. In particular, an exciter design and system that is capable of supporting a wide range of carrier frequencies is disclosed.